A protein extracted from Phytolacca americana (PAP) known to inhibit RNA virus replication, has been shown to enzymatically disrupt the ribosomal binding site for elongation factors used in protein synthesis. The characteristics of this disruption indicate that PAP probably has an enzymatic mechanism very similar to that of the anti-tumor proteins, Ricin, RCA II and Abrin. We have obtained large single crystals of PAP which diffract to 2.0A resolution. We propose to solve the three dimensional structure of this toxin by x-ray crystallography. Our work should mesh with ongoing biochemical analysis of the mechanism of action of PAP, and with the amino acid sequence now underway for PAP, to produce a detailed understanding of the mechanism of action of anti-viral and anti-tumor toxins.